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Straight answers to the questions people ask when choosing, fitting and looking after wire feed rolls for a MIG machine.
The groove shape is chosen to suit the wire. V-groove rolls are the usual choice for hard solid wires such as mild steel and stainless steel. Knurled V-groove rolls are normally used for flux-cored and metal-cored wire, which has a hard sheath and needs extra grip. U-groove rolls spread the pressure on soft wire such as aluminium so it is not crushed.Using the wrong shape is a common cause of slipping, deformed wire and poor arc stability, so check the groove against the wire before looking for other faults.
Each groove is made for a particular wire diameter, so the roll must match the wire you are feeding. A mismatch can cause slipping, an erratic arc or wire that is deformed or shaved on its way through.The size is normally marked on the roll or listed in the machine's parts list, so check the manual for your feeder. Some rolls have a different groove on each face, so make sure the groove you want is on the outside, facing the wire, when you fit it.
A two-roll feeder has one pair of rolls gripping the wire and is common on lighter-duty machines. A four-roll feeder has two pairs, giving a larger area of grip, so less pressure is needed at any one point and the wire is less likely to be deformed.Four-roll feeders are generally preferred for heavy industrial use and for soft wires, but two-roll feeders also feed a range of wires. This depends on the machine, so check the manual for the rolls your feeder takes.
Too little pressure lets the wire slip in the rolls, while too much flattens or deforms the wire and causes feeding problems and an unstable arc. Aim for the lowest pressure that feeds the wire reliably.One common method with a cold torch is to loosen the pressure until the wire slips, then tighten in half-turns until it feeds without slipping, either checking against a wooden block or by holding the wire with a gloved hand. Follow the maker's instructions for your feeder, and wear gloves as the wire can be hot.
Feed rolls are wear parts and knurled rolls in particular wear out, so inspect them regularly. Worn rolls become less effective at moving the wire, so you may see erratic feeding or slipping even when the pressure is set correctly.Do not simply tighten the pressure to cover slipping, as this can flatten the wire and wear the liner and tip faster. There is no fixed replacement interval, as it depends on the wire, the amount of welding and the settings.
It can be, but check the whole wire path. Look at whether the groove type and size match the wire, whether the rolls are worn and whether the pressure is sensible. Then check the parts after the rolls: a worn or dirty liner, a contact tip that is the wrong size and tight bends in the cable all add load.A quick check is to see how smoothly the wire pulls through the torch with the pressure released. Heavy drag points to the liner or tip rather than the rolls, and a liner that needs force to pull wire through should be replaced.